Some electrical and electronic circuits generate undesirable electromagnetic emissions and some circuits are adversely affected by electromagnetic emissions from other circuits. There are various causes for electromagnetic emission and for sensitivity to emissions. Current levels, conductor lengths, inductance, high frequency operation, and high slew rates (e.g., from switching inductive loads) contribute to generating electromagnetic interference or “EMI.” High amplifier gains, low common mode rejection, low signal to noise ratios and other factors contribute to circuit sensitivity to incoming noise.
One technique for blocking the propagation of emissions, either incoming or outgoing, is to place a conductive barrier such as a metal sheet or screen, across the propagation path. An incident electromagnetic signal induces current in the conductive barrier, but the propagating signal is attenuated, particularly if the barrier is connected along a low resistance path to the applicable signal ground reference.
Therefore, problematic sensitive and/or high frequency circuits are often shielded by grounded conductive barriers. In one example, box-like sheet metal structures wholly or partly enclose around such circuits. In other examples, enclosures are formed of conductive polymers, or coated with conductive foils or conductive paints. The enclosures can be rigid or flexible, and can be box shaped, cylindrical, domed or otherwise shaped to define an enclosure of conductive material around the applicable circuit.
Shielding panels need not be wholly continuous, and there may be cooling interests as well as shielding interests. A conductive enclosure around a circuit can have holes or slots, or can be constructed using a conductive mesh screen. Shields having holes or slots or other openings can be effective as conductive barriers for frequencies at and below a characteristic frequency related to the size of the holes. For relatively higher frequency shielding, any holes through the shield barrier need to be relatively smaller, etc.
In connection with a printed circuit board arrangement, a rectilinear sheet metal box often is used as all or part of the shield barrier. A box can be formed by folding an integral sheet and/or attaching together two or more integral sheets so that the sheets together form a conductive barrier in the required shape.
Assuming the example of a rectilinear box on a printed circuit board, conductive portions of the board can define part of the barrier around a given circuit element, or barriers can be provided on opposite sides if necessary. On a given side, standing conductive walls of thin sheet metal can extend from the plane of the circuit board, e.g., extending perpendicularly upwardly from a folded flange attached to the board, or carried by one or more integral tabs that engage openings on or through the board as attachment pegs, feet, floor panel elements or the like.
The standing wall elements act as the panels of a fence defining a perimeter and keeping the EMI emissions of the circuit in or out. The panels need to be mechanically mounted to remain in position, and electrically connected to one another and to one or more points on the circuit board, typically a common ground point.
Many of the same shielding concerns that are involved in forming an enclosure around elements on a printed circuit card are encountered in building cabinets for apparatus. For example, shielding is advantageous and may be required by regulatory authorities, for equipment such as computer cabinets (e.g., PCs, servers, mainframes, etc.), peripherals (printers, scanners, modems), radio frequency transmission and reception devices (receivers, television sets) and the like. In addition to enclosing sensitive elements on circuit cards, circuit subsets may need enclosures (such as power supplies, disk drives and other devices that are internal to a cabinet). Cabinet structural panels need to be attached, and various ports that might permit propagation of electromagnetic fields, such as backplane circuit connector openings, need to be bridged over or plugged by a conductive cover. Although designers sometimes opt for relatively permanent attachments, it is advantageous if provisions are made whereby such shielding can be disassembled and reassembled or perhaps varied to accommodate different specific combinations of circuit elements in a cabinet or on a circuit card, e.g., to provide connector access to expansion card slots on a computer motherboard.
The need for effective electromagnetic shielding suggests that conductive parts of a shield should be securely affixed, both electrically and mechanically. On the other hand, convenience of assembly and controlling the cost of shielding parts may lead toward less secure or durable structures. What is needed is a good balance of interests.
Shield enclosures typically comprise inexpensive thin sheet metal stampings of aluminum, stainless steel or another material. There may be instances where it is desirable, e.g., for compactness and to reduce internal enclosure dimensions, to provide a shield with a complicated shape, but more often it is desirable to minimize expense by using a minimum of stamping steps, to produce a simple structure such as a box, a cylinder with an end cap, a covered rectangular opening or the like.
An exemplary simple structure could resemble the walls and lid of a shoe box, with a rectilinear perimeter of standing walls and a lid. To provide a connection and facilitate disassembly if necessary, the lid can have resiliently biased gripping structures overlapping the top edges of the side walls.
U.S. Pat. No. 6,552,261—Shlahtichman et al. discloses a push-fit shield enclosure comprising a box with walls covered by a lid having spaced fingers for resiliently gripping over the outside edges of the box walls. U.S. Pat. No. 5,354,951—Lange Sr. et al. discloses an arrangement in which finger-like projections of the walls grasp resiliently around the outer edges of a lid panel. Shield enclosures built by assembling resilient grasping structures are convenient to assemble and can be readily disassembled if necessary, but they are not very secure.
U.S. Pat. No. 6,005,186—Bachman discloses a shield plug that can be fit into an opening in a computer back panel to seal a circuit card connector opening. The plug has resiliently deformable depressions that are intended to become depressed by the edges of an opening in a cabinet wall when the plug is inserted to close the opening. The deformable depressions are spaced from associated flanges by a distance approximately equal to the thickness of the cabinet wall. When the plug is installed, the depressions are resiliently compressed in passing through the opening, and snap outwardly again as the plug is fully seated, holding the plug securely in place. A similar snap fit for affixing the lid of a shielding box structure is disclosed in U.S. Pat. No. 5,495,399—Gore et al., wherein resilient depressions and complementary openings are placed in the perimeter walls of a shield box and in the overlapping flanges of a box lid.
Such structures ensure that the lid (or plug or other filling structure) become securely affixed. However, they are somewhat difficult to disassemble. Disassembly that involves prying apart engaged snap fittings and the like often results in damage to the shield structures or to the shielded parts.
It would be desirable to provide a technique whereby simple shielding enclosures could be made without the need for great precision in the size and shaping of the parts, so as to minimize expense. At the same time, however, such shielding enclosures need to be mechanically secure as well as providing dependable low resistance electrical connections. On the other hand, provisions should be made for convenient disassembly when necessary, without substantially compromising the security of the connections in normal use.